1. Field of the Invention
The present invention relates generally to chemical mechanical planarization (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to a hollow annular gimbal ring having internal gel suitable for providing gimbal movement of a wafer carrier plate relative to a carrier head.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. At each metallization level and/or associated dielectric layer, there is a need to planarize the metal and/or dielectric material. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove overburden materials, such as copper metallization.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to polish, buff, and scrub one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation, and may be distributed by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
In a typical CMP system, a wafer is mounted on a carrier head, which rotates in a direction of rotation. The CMP process is achieved when an exposed surface of the rotating wafer is applied with force against a polishing pad, which moves or rotates in a polishing pad direction. Some CMP processes require that a significant force be used at the time the rotating wafer is being polished by the polishing pad.
Normally, the polishing pads used in the CMP systems are composed of porous or fibrous materials. Depending on the type of the polishing pad used, slurry composed of an aqueous solution containing different types of dispersed abrasive particles such as SiO2 and/or Al2O3 may be applied to the polishing pad, thereby creating an abrasive chemical solution between the polishing pad and the wafer.
FIG. 1A depicts a schematic cross-sectional view of an exemplary prior art CMP system. In this CMP system a carrier head 100 engages a retaining ring mounting plate 101 provided with a retaining ring 102. The retaining ring 102 centers a wafer 103 relative to a vertical axis of rotation 104 of the carrier head 100. The carrier head 100 is urged toward a surface 106 of a polishing pad 107 with a force F. As shown, an outer surface 108 of the retaining ring 102 is positioned above an exposed surface 109 of the wafer 103. Thus, while the exposed surface 109 of the wafer 103 is in contact with the polishing pad surface 106, the outer surface 108 of the retaining ring 101 is configured to not come into contact with the polishing pad surface 106, and is thus spaced from the polishing pad surface 106. The spacing of the ring 102 from the surface 106 allows room for the wafer 103, the mounting plate 101, and the ring 102 to tilt relative to the vertical axis 104 on which the carrier head 100 rotates. A typical gimbal 111 is provided as a spherical member 112 mounted in spherical sockets 113a and 113b of the respective carrier head 100 and mounting plate 101. One or the other of the sockets 113a or 113b is configured to secure the member 112 to the respective carrier head 100 or mounting plate 101.
FIG. 1B shows the tilt of the wafer 103, the mounting plate 101, and the retaining ring 102 allowed by the gimbal 111 in terms of an angle 116 between the vertical axis 104 and an axis of rotation 117 of the retaining ring mounting plate 101. The tilt allows movement of the mounting plate 101 for parallelism of a plane (represented by a line 118) of the exposed surface 109 of the wafer 103 and a plane (represented by a line 119) of the surface 106 of the pad 107.
Several problems may be encountered while using an xe2x80x9cedge-effectxe2x80x9d caused by the CMP system polishing the edge of the wafer 103 at a different rate than other regions, thereby creating a non-uniform profile on the surface of the wafer 103. The problems associated with edge-effect are twofold, namely xe2x80x9cpad rebound effectxe2x80x9d and xe2x80x9cedge burn-off effect.xe2x80x9d FIG. 1C is an enlarged illustration of the pad rebound effect associated with the prior art. The pad rebound effect occurs when the polishing pad surface 106 initially comes into contact with the edge of the wafer 103, causing the polishing pad surface 106 to bounce off the exposed surface 109 of the wafer 103. As the moving polishing pad surface 106 shifts under the exposed surface 109 of the wafer 103 (see arrow 120), the edge of the wafer 103 cuts into the polishing pad 107 at an edge contact zone 121. The cutting causes the polishing pad 106 to bounce off the wafer 103, thereby creating a wave on the polishing pad 106 as shown in FIG. 1C. Ideally, the polishing pad 107 is configured to be applied to the wafer 103 at a specific uniform pressure and to remain flat (planar). However, FIG. 1C shows that the wave created on the polishing pad 103 creates a series of low-pressure regions of the exposed surface 109 of the wafer 103. Such regions may include an edge non-contact zone 122 and an inner non-contact zone 123, wherein the removal rate is lower than the average removal rate. Thus, the edge contact zone 121 and an inner contact zone 124 of the wafer 103 are polished more than the other zones. As a result, the CMP processed wafer 103 will tend to show a non-uniform profile.
Further illustrated in FIG. 1D is the xe2x80x9cedge burn-offxe2x80x9d effect. As the polishing pad surface 106 comes into contact with the sharper edge of the wafer 103 at the edge contact zone 121, the edge of the wafer 103 cuts into the polishing pad 107, thereby creating an area defined as a xe2x80x9chot spot,xe2x80x9d wherein the pressure exerted by the polishing pad 107 is higher than the average polishing pressure. Thus, the polishing pad surface 106 excessively polishes the edge of the wafer 103 and the area around the edge contact zone 121 (i.e., the hot spots). By the burn-off effect, a substantially high removal rate is exhibited at the area within about 1 millimeter to about 3 millimeters of the edge of the wafer 103. Moreover, depending on the polisher and the hardware construction, a substantially low removal rate is detected within the edge non-contact zone 122, an area between about 3 millimeters to about 20 millimeters of the edge of the wafer 103. Accordingly, as a cumulative result of the edge-effects, an area of about 1 millimeter to about 20 millimeters of the edge of the resulting post-CMP wafers 103 sometimes could be rendered unusable, thereby wasting silicon device area.
One way to compensate against edge effects is to use a gimbal, such as the gimbal 111. However, such gimbals 111 also suffer problems in that the complexity of the mechanical components of such gimbals makes them difficult to design and implement for symmetric repetitive CMP environments. For example, some typical gimbals 111 tend to vibrate in response to the forces of the polishing pad 107 and the wafer 103. The vibrations may introduce numerous potential problems to troubleshot when inappropriate CMP results start appearing in processed wafers 103. Thus, the vibrations may be difficult to reproduce or analyze, making it difficult to eliminate the inappropriate CMP results.
In view of the foregoing, a need exists in the art for a chemical mechanical planarization system that substantially eliminates damaging edge-effects and their associated removal rate non-uniformities. Such need includes provision of an improved gimbal that is subject to reduced vibrations and that simplifies the design of the carrier head 100.
Broadly speaking, the present invention fills these needs by providing a chemical mechanical planarization (CMP) system having a carrier body for applying force along a central axis and a retainer body for holding a wafer centered on the axis, wherein the carrier body and the retainer body each define a perimeter edge and a plane. The planes are separated by a space having a uniform dimension when the planes are parallel and having a non-uniform dimension when the planes are not parallel. An active gimbal is received in the space and configured with a hollow annular body having an arcuate wall structure extending around and radially spaced from the central axis. The wall structure is configured with a first section generally at one side of the axis and adjacent to the perimeter edges and with an opposite section generally at an opposite side of the axis and adjacent to the perimeter edges. With the active gimbal in the space separating the planes, the first and second sections of the arcuate wall structure are unevenly deformed when the space has the non-uniform dimension. To complete the active gimbal a gel-like material fills the hollow wall structure so that when the first and second sections are unevenly deformed a portion of the gel is caused by the deformed first section to flow in the hollow annular body from the one side of the axis to the second section to fill the deformed second section with the gel while allowing the deformed first section to remain filled with another portion of the gel. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, an active gimbal allows the space between the carrier plate and the wafer support to vary in configuration as the carrier plate and the wafer support plate tilt during CMP processing. The active gimbal may be configured as a hollow toroidally-shaped body for reception in the carrier plate-wafer support space between the carrier plane and the support plane. The body has an arcuate wall structure extending around and radially spaced from a co-axial center of the carrier plate and the wafer support plate. The toriodally-shaped body is configured so that a diameter of the body divides the body into a first half that is squeezed when the planes are not parallel and into a second half that is permitted to expand when the planes are not parallel. The first and second halves are configured to encompass substantially equal volumes when the planes are parallel and to encompass substantially unequal volumes when the planes are not parallel. The sum of the unequal volumes substantially equals the sum of the equal volumes. A gel, or gel-like material, is received in and fills the hollow toroidally-shaped body. The gel-like material is substantially incompressible so that as the first and second halves of the hollow body encompass the substantially unequal volumes, the gel-like material is caused to flow in the hollow body from the first half to the second half to fill the encompassed unequal volumes. The gel-like material in the unequal volumes serves to maintain the hollow toroidally-shaped body in contact with each of the carrier plane and the support plane when the planes are not parallel.
In still another embodiment, a method is provided for making a gimbal for use in a CMP carrier head. The gimbal is configured to be positioned between the carrier head and a wafer carrier. A first operation selects an elastomeric material having compression and decompression characteristics suitable for response to CMP forces. Another operation configures a hollow annular ring for reception in a cavity defined in opposed spaced surfaces of the carrier head and the wafer plate. The ring is configured from the selected elastomeric material. Another operation selects a gel-like material having a viscosity suitable for dampening the CMP forces applied to the carrier head and the wafer plate. A final operation fills the hollow annular ring with the selected gel-like material.
In yet another embodiment, there is a method of making an active gimbal for allowing the wafer support plate to tilt with respect to the carrier plate during CMP processing. An operation of the method configures a hollow body for reception in the space between the carrier plate and the wafer support. The configuring provides the body with a toroidal shape defined by an arcuate wall structure extending around and radially spaced from a co-axial center of the carrier plate and the wafer support. The toriodally-shaped body is configured so that a diameter of the body divides the body into a first half and a second half. The first half is squeezed when planes defined by the respective carrier plate and wafer support are not parallel. The second half is permitted to expand when the planes are not parallel. This configuring allows the first and second halves to encompass substantially equal volumes when the planes are parallel and to encompass substantially unequal volumes when the planes are not parallel. In a filling operation, the hollow toroidally-shaped body is filled with a gel-like material that is substantially incompressible. The filled hollow toroidally-shaped body is placed in the space between the carrier plate and the wafer support. As the first and second halves encompass the substantially unequal volumes when the planes are not parallel, a portion of the gel-like material is caused to flow in the hollow body from the first half to the second half to fill the encompassed unequal volumes. The gel-like material in the unequal volumes maintains the hollow toroidally-shaped body in contact with each of the carrier plane and the support plane when the planes are not parallel.
In still another embodiment, the filling operation is performed with the gel-like material having a viscosity greater than that of water to impede but still permit flow of the gel-like material from a smaller of the unequal volumes to a larger of the unequal volumes when the planes not parallel.
The advantages of the present invention are numerous. Most notably, the active gimbal of the present invention is easy to make and assemble with the carrier plate and the wafer support. Furthermore, once the material for the hollow body has been selected, the deformation of the hollow body in response to equal forces does not change over time, as the selected elastomer material retains the characteristic of returning to its original undeformed shape. Also, once the material for the gel-like material has been selected, the flow of the gel-like material within the hollow body does not change over time in response to the same forces of the hollow body, because the viscosity of the selected gel-like material remains the same. The gel-like material thus serves to maintain the hollow body in contact with each of the carrier plane and the support plane as the planes move into and out of parallel. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.